An alien wavelength is a DWDM channel provisioned on the DWDM line system to provide layer 0 physical connectivity between two DWDM capable devices. These devices can be a standalone DWDM transponder like the NCS-1004 or a router using DWDM pluggable optics. The term "alien" means that the DWDM line system has no awareness of the DWDM signal source device. With the correct wavelength and optical power coming from the alien device, the line system will establish the connection.
Alien wavelengths have become more common in DWDM networks because customers want the flexibility to transport various DWDM devices from multiple vendors with their DWDM line systems. Until the introduction and adoption of alien waves, DWDM networks were closed, vendor-specific systems requiring both the line system and transponder within the same platform.